1. Field of the Invention
This invention relates generally to the calibration of pulse height analyzers and pertains in particular to improved methods and apparatus for enabling the energy-channel calibration of a pulse height analyzer to be maintained over a wide temperature range.
2. The Prior Art
Of the many well logging instruments and techniques developed over the years to determine the hydrocarbon content and productivity of earth formations, the spectroscopy tool, by which energy spectra of the constituents of formation matrices and fluids are generated, has been found to provide information of particular value in formation analysis. Typically, the energy spectra are obtained by irradiating the formations of interest with pulses of high energy neutrons and detecting the gamma rays resulting from interactions of the neutrons with the formation nuclei in a way which converts each detected gamma ray into an electrical pulse whose amplitude is a measure of the gamma ray energy. These pulses are then sorted according to height in a pulse height analyzer to develop energy spectra characteristic of the constituents near the tool.
The quality of gamma ray energy spectra thus provided is of course dependent upon the stability of the energy response of the detector-analyzer system during use. This is of particular concern in well logging, where environmental conditions are severe, and especially so in tools in which the pulse height analyzer is located downhole. In such cases, instabilities in the energy calibration of the detector-analyzer system commonly result, for example, as a direct effect of changes in well bore temperature or indirectly from temperature-induced and other fluctuations in power supply voltages for the downhole electronics. Such instabilities frequently manifest themselves as variations in the pulse amplification (gain) of the detector-analyzer circuitry or in the zero offset of the linear pulse height-channel relationship of the pulse sorting circuitry of the pulse height analyzer. "Zero offset" as used hereinafter refers to the output of the analyzer for a zero energy input. These problems are particularly troublesome in spectroscopy systems intended for analysis of gamma rays of energies substantially higher than those of available natural calibration sources, i.e., where the ratio of the full scale energy of the system to the calibration energy is large, and where the resolution quality of the system does not admit of the use of induced gamma ray peaks for calibration purposes. Desirably, therefore, some provision is made in such systems for examining the response of the system during use and for compensating for any instabilities detected. Various apparatus and techniques have been proposed for this purpose in the past. These apparatus and techniques, however, have not fully responded to the requirements of the art in this regard.
One prior art technique for stabilizing a multi-channel pulse height analyzer is to enclose it in an oven which has a higher temperature than the borehole, as is described in U.S. Pat. No. 3,534,401 to Karvellas et al. Naturally this is inefficient because of the unduly large power requirements. If the pulse height analyzer has only a few channels, such as the ones shown in U.S. Pat. Nos. 3,264,475; 3,368,075 and 3,394,256 to Reed et al., only the threshold voltage of each channel need be maintained and not the gain and offset. Among other disadvantages, however, this technique is not practicable for analyzers having numerous channels. Furthermore, Reed et al. make no provision for detecting and compensating for changes in the operating characteristics of circuit components due to temperature gradients within the borehole.